examples/networking: sockmap.py: Fix invalid bpf_context access off=24 size=8

[Rtoax]

clang generates such patterns occasionally when it thinks only up to specific size is accessed anyway. Fix the following error:

; .remote_ip4 = skops->remote_ip4, @ main.c:29
7: (79) r2 = *(u64 *)(r1 +24)

Error detail:

$ sudo ./sockmap.py -c /path/to/cgroup/foo
bpf: Failed to load program: Permission denied
0: R1=ctx() R10=fp0
; if (skops->family != AF_INET) @ main.c:55
0: (61) r2 = *(u32 *)(r1 +20)         ; R1=ctx() R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
1: (55) if r2 != 0x2 goto pc+88       ; R2_w=2
; u32 op = skops->op; @ main.c:52
2: (61) r2 = *(u32 *)(r1 +0)          ; R1=ctx() R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
3: (18) r3 = 0xfffffffe               ; R3_w=0xfffffffe
; switch (op) { @ main.c:58
5: (5f) r2 &= r3                      ; R2_w=scalar(smin=0,smax=umax=umax32=0xfffffffe,smax32=0x7ffffffe,var_off=(0x0; 0xfffffffe)) R3_w=0xfffffffe
6: (55) if r2 != 0x4 goto pc+83       ; R2_w=4
; .remote_ip4 = skops->remote_ip4, @ main.c:29
7: (79) r2 = *(u64 *)(r1 +24)
invalid bpf_context access off=24 size=8
processed 7 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

Traceback (most recent call last):
  File "/home/sda/git-repos/iovisor/bcc/examples/networking/./sockmap.py", line 110, in <module>
    func_sock_ops = bpf.load_func("bpf_sockhash", bpf.SOCK_OPS)
  File "/usr/lib/python3.13/site-packages/bcc/__init__.py", line 526, in load_func
    raise Exception("Failed to load BPF program %s: %s" %
                    (func_name, errstr))
Exception: Failed to load BPF program b'bpf_sockhash': Permission denied

[yonghong-song]

Thanks. This is indeed a problem. Unfortunately as you mentioned, clang may do load/store for adjacent 2 32-bit fields. Another option is to do separate loads for remote_ip4 and local_ip4 and add an asm memory barrier between them. Of course you fix also works.